The present invention relates to dissipating stored energy in an inductive load and, more particularly, to dissipating stored inductive energy in an inductive load that is coupled to an integrated circuit system. Further, the invention pertains to safely dissipating the energy stored in an inductive load of an automotive integrated circuit system to prevent damage to the system under certain conditions.
Automotive systems such as window control systems, fuel injector systems, fuel pumps, etc. are comprised of solenoids or relays for controlling the operation of the particular system. The relays or solenoids may be driven or energized by an integrated control module circuit comprising semiconductor devices. The integrated control module circuit may include a PNP transistor connected in a series pass configuration between a source of operating potential (a battery) and the inductive load (a relay or solenoid). The turn-on and turn-off of the PNP transistor is controlled by internal control circuitry of the control module. Thus, the PNP transistor functions as a current switch for energizing and de-energizing the inductive load. Generally, these inductive loads are located distant from the integrated control module circuit although both are connected to a common electrical ground reference which may be the automotive chassis.
During operation, the inductive loads are driven from the positive side of the battery via the PNP transistor switch. As the PNP transistor is turned off, the inductive load will attempt to maintain the current flow therethrough as is understood. This condition causes a large negative voltage transient to occur at the collector of the series pass PNP transistor. In at least one prior art system an external discrete power Zener diode is connected between the positive battery supply and the collector of the series pass transistor. The Zener is rendered conductive in response to the negative transient voltage to provide a current conduction path to the inductive load as the series pass transistor turns off so that the stored energy of the load is dissipated whereby the integrated control module circuit is protected since the Zener limits the negative voltage to a non-distructive maximum level.
A problem with the above dissipation method and system is that the Zener diode must supply relative large currents to the inductive load in combination with a large voltage drop thereacross which means that the Zener must be a high power device. Thus, a large external discrete device is required in order to be capable of handling the power that must be dissipated. Additionally, state of the art power Zeners are relatively costly to the automotive manufacturer.
Another problem with prior art automotive systems using the Zener diode protection described above relates to reverse battery conditions. For instance, if the battery of the automobile is connected in reverse, i.e., the negative and positive terminals being reversed, a negative potential is supplied to the control module and the zener. This condition causes the Zener to be forward biased to provide a direct current conduction path to the inductive load. If the inductive load is, for example, a relay it will be energized since most, if not all, relays are bilateral, i.e., they are energized by current flowing in any direction. This is a very undesirable condition as the relay may drive a fuel pump that would therefore be continuously operated as the battery is connected in reverse. This problem is sometimes solved in prior art systems by the addition of a diode in series with the Zener to block conduction during reverse battery conditions. This solution adds another component which is costly to the manufacturer.
In addition to the above described reverse battery condition, the automatic integrated control module must also withstand other abnormal conditions such as the loss of the positive battery connection or the loss of the ground connection during the time that the inductive load is energized. Some prior art protection means such as the Zener or Zener plus diode methods described above, do not provide a conduction path for the inductively stored energy for both of these abnormal conditions. Thus, the high negative voltage that develops across the inductive load can exceed the destructive level of the integrated circuitry within the control module.
Hence, there exists a need for an improved and inexpensive system for dissipating stored inductive energy in an inductive load coupled to an integrated circuit, especially when such integrated circuit is utilized in the automotive environment.